A known and accepted circuit element in the integrated circuit industry is the bipolar junction transistor (BJT). A popular and widely accepted BJT is known as the diffused BJT. In a diffused BJT, a diffusion region is formed that functions as a collector for the BJT. A base diffusion is formed within the collector diffusion, and an emitter electrode diffusion is formed within the base diffusion.
Another BJT that is known and accepted in the integrated circuit industry is the single polysilicon BJT. In the single polysilicon BJT, a deposited polysilicon layer is used to form an emitter electrode and/or an emitter contact. A diffusion is used to form a base electrode adjacent the emitter electrode, and a deep collector is formed by a buried doped region which underlies the base and emitter electrodes.
Yet another BJT which is known and accepted in the integrated circuit industry is the double polysilicon BJT. In the double polysilicon BJT, a first deposited polysilicon layer is used to form an emitter electrode and/or an emitter contact. A second deposited polysilicon layer is used to form a base electrode portion adjacent the emitter electrode. A deep collector is formed by a buried doped region which underlies the base and emitter electrodes in a manner which is similar to the single polysilicon BJT.
In the above embodiments, electrical contacts are formed to each of the emitter, base, and collector by forming one or more conductive layers over each of either the emitter, base, and collector by either deposition or a sputtering process. The contacts are therefore not self-aligned in some cases and result in structures that are less than optimal in terms of surface area. Photolithographic alignment and alignment tolerances also contribute to increased surface area when using this technology.
In order to self-align various features of a BJT, epitaxially grown emitter electrode contacts have been used. These epitaxially grown emitter electrode contacts are grown only from the substrate. In some cases, the emitter electrode contacts are grown in a manner that allows for the contact material to laterally overgrow a top portion of an oxide layer wherein the oxide layer is positioned adjacent the emitter electrode contacts. The lateral overgrowth of the emitter electrode contact is used to form a self-aligned contact pad for the emitter electrode. By initiating the growth from the substrate surface only, a long growth time is required for overgrowth to occur. Therefore, the epitaxial step, which is usually performed at temperatures of 850.degree. C. or higher, contributes greatly to a thermal budget of the overall integrated circuit process. A large thermal budget results in deeper diffused electrode regions that will, in most cases, increase parasitic capacitance and resistance and degrade bipolar transistor performance.